This invention relates generally to machines to facilitate the mounting of rubber printing plates and for obtaining proofs thereof, and more particularly to a mounting-proofing machine of exceptionally simple design capable of handling plate cylinders in a large range of diameters.
In the flexographic process, printing is effected by rubber printing plates mounted on cylinders, the paper to be printed being impressed on the inked printing plate. The cylinder on which the printing plates are mounted is generally called the plate cylinder. The quality of a flexographic printing job depends, in large measure, on the care with which pre-press preparations are carried out. Plate-mounting, color registration and proofing are effected off the press by means of commercially available mounting-proofing machines designed for this purpose.
These machines, which usually make use of an optical mounting system, make it possible to mount the plates on plate cylinders to effect exact color-registration, a procedure essential to the maintenance of both quality and economy in all flexographic operations. Pre-proofing is, in many respects, the most important of all pre-press preparations, for it not only indicates the appearance of the final reproduction, but it also affords means to check the mounting of the plates for color sequence, spacing requirements, layout and gear size, as well as copy and color separation.
Mounting-proofing machines are provided with a proofing cylinder (sometimes called the impression cylinder) which cooperates with the plate cylinder, the proofing cylinder making contact with the printing plates on the plate cylinder and rotating concurrently therewith to print a proof on a sheet secured to the proofing cylinder. In commercial machines of the type heretofore known which make use of optical mounting techniques, the proofing or impression cylinder is supported for rotation at a fixed position, whereas the plate cylinder is movable, usually in the vertical direction, from a mounting state in which it is retracted relative to the proofing cylinder to a proofing state in which it is in engagement therewith.
The proofing and plate cylinders are mechanically intercoupled, whereby rotation of the proofing cylinder causes the plate cylinder to rotate. When the diameter of the proofing cylinder is the same as the printing diameter of the plate cylinder (i.e., the diameter of the plate cylinder plus the thickness of the printing plates thereon), then a one-to-one relationship exists therebetween.
But since in practice the plate cylinders come in a range of diameters for printing different print lengths, it has heretofore been necessary to adjust the phase relationship between the plate and proofing cylinders to accommodate the differences between the cylinder diameters. For adjusting this phase relationship for different plate cylinder diameters, a relatively complex mechanism is required in existing types of mounting-proofing machines.
Another drawback of existing types of mounting-proofing machines is their limited capacity to handle plate cylinders of different diameter. With machines of the type heretofore known, the capacity of the machine is restricted to a range of plate cylinder diameters extending from about 95 percent of the diameter of the proofing cylinder down to about 25 or 30 percent thereof, or approximately 4 to 1. Moreover, since in existing structures, the proof forces imposed at contact are eccentrically-opposed the structures required to accommodate these magnifield forces are too large to permit smaller sizes of plate cylinders to fit the machine.